The upper piston part usually has annular profiles and/or annular flanges, for example a sealing cone, for the abutment and airtight connection of the air spring rolling-lobe flexible member on/to the roll-off piston.
Various types of such roll-off pistons are known in the prior art. On the one hand, there are relatively heavy roll-off pistons of sheet steel with a usable interior volume, which are screwed or welded to the corresponding connection parts. The roll-off piston of sheet steel is produced as a deep-drawn part with a conical sealing seat for receiving the rolling-lobe flexible member and is accordingly heavy and expensive to produce.
On the other hand, there are relatively lightweight one-part plastic pistons with an interior volume that is not used or only partially used and molded-on or embedded fastening parts of metal, with which a connection to the connection parts can be realized. A plastic piston with a completely used interior volume is disclosed by EP 1 862 335 B1. Described there is an air spring piston which consists of a cup-shaped part and a cover part, which are butt-welded in the region of their walls.
United States patent application publication 2010/0127438 discloses a plunger piston for an air spring that is formed as a hollow body and has two parts connected to one another in an airtight manner, to be specific a pot-shaped lower part, having a base and a casing, and an upper part.
Proceeding from the known air springs with a roll-off piston made of steel, pistons made of glass fiber reinforced plastic (GRP), for instance polyamide PA 66 GF 30 have been developed with the aim of lowering weight and the costs involved in producing the pistons. However, the advantages of this material are also offset by disadvantages. These are that the material is relatively hard and brittle and can splinter if it breaks as a result of fatigue or mechanical overloading.
In particular whenever a roll-off piston consisting of glass fiber reinforced plastic is damaged, it is possible that the material will break brittlely, with the possibility of producing sharp-edged fragments and splinters.
In the case of roll-off pistons of which the interior volume is used for enlarging the working space of the air spring, the resultant risk increases, because the internal pressure can accelerate the splinters produced.
Typically, plastic pistons are produced by injection-molding processes. Such a piston produced as an injection-molded part may have weak points as a result of flow fronts, which in the case of pistons with a fully used interior volume often lie in the zones subjected to particularly high loading between the reinforcing ribs.
Moreover, at present, pistons with a fully used interior volume and conical sealing with respect to the rolling-lobe flexible member can usually only be produced as pistons of two parts that are connected to one another by friction welding. The friction weld is a point at which discontinuities of the material parameters can likewise occur, for example also because in the case of glass fiber reinforced plastics the glass fibers in the weld are not aligned, and therefore have less of a strengthening effect.